As is known in the art, tomography is often used in construction of detailed images of internal structures of objects. Tomography relies upon a selected form of energy being directed toward and passing through the object at more than one angle. The energy from the various angles is collected and processed to provide a tomographic image. The received signals are typically less intense (i.e., darker) where the object is thicker or more dense, and more intense (i.e., brighter) where the object is thinner or less dense.
As is known, a signal received by a single energy sensor (i.e., at one angle) does not contain sufficient information to generate either a two-dimensional or a three-dimensional representation of internal structures of the object. As is also known, signals received by energy sensors arranged in a plane or volume provide sufficient information to generate a three-dimensional representation of internal structures of the object.
Tomography is used in a variety of systems with a variety of types of transmitted and received energy. For example, in x-ray Computed Axial Tomography (CAT), x-ray energy is projected through an object, typically at a variety of angles, and a variety of x-ray receivers, at a corresponding variety of angles, are used to receive the x-ray energy. A computer is used to generate an image of internal structures of the object in three dimensions from signals received by the variety of x-ray receivers. It should be recognized that x-rays tend to pass through the object in straight lines with relatively little attenuation.
One type of x-ray CAT system is used for medical imaging and the object through which the x-rays are projected is a person. However, the x-ray CAT system can be used to image internal structures of other objects, for example, luggage at an airport.
Some forms of optical tomography are known, which use one or more wavelengths of visible or invisible light rather than x-rays. However, unlike x-ray tomography, for which x-rays tend to pass through an object in a straight line with relatively little attenuation, light tends to be absorbed and to scatter when passing though an object. Therefore, light does not travel in straight lines when passing through the object. Light also tends to attenuate and to scatter more when passing though a relatively thick object having a relatively non-homogeneous medium, than it tends to attenuate and to scatter when passing through a relatively thin object having a relatively homogeneous medium.
Diffuse Optical Tomography (DOT) and Fluorescence Molecular Tomography (FMT) are known optical imaging techniques that allow optical tomography imaging of internal structure of body parts of animals and humans. DOT is an effective imaging technique capable of imaging hemoglobin concentration and oxygenation. DOT can increase the specificity in detecting disease when used in combination with more established examinations (for example for cancer or arthritis detection and characterization). In addition, DOT is used to study brain activation and to study exercising muscle.
FMT uses fluorochromes, which absorb light propagating inside of an object and emit light at a longer wavelength (lower energy) than the absorbed light inside of the object, allowing non-invasive in-vivo investigation of functional and molecular signatures in whole tissues of animals and humans. FMT enables molecular imaging, i.e., it can probe molecular abnormalities that are the basis of a disease, rather than imaging the end-anatomical effects of the molecular abnormalities as with conventional imaging approaches. Specific imaging of molecular targets provides earlier detection and characterization of a disease, as well as earlier and direct molecular assessment of treatment efficacy. FMT technology can also transfer typical in-vitro fluorescence assays, such as gene-expression profiling, elucidating cellular pathways or sensing small molecule protein interaction to in-vivo non-invasive imaging applications of large tissues.
Some conventional optical tomography systems use near infrared (near-IR or NIR) light, instead of light in the visible spectrum when passing through animal tissues, since NIR tends to attenuate less than visible light. The use of NIR light instead of light in the visible spectrum provides the ability to image deeper tissues, i.e., thicker tissues, or with higher sensitivity than in the visible light region. The development of highly efficient fluorescent probes, i.e., appropriately engineered fluorochromes with high molecular specificity emitting in the NIR, has also enabled FMT imaging of deeper tissues.
Mathematical modeling of light propagation in animal tissue and technological advancements in light sources (photon sources) and light sensors (photon receivers or photo detectors) has made optical tomography possible using diffuse light. Diffuse Optical Tomography (DOT) uses multiple projections and de-convolves the scattering effect of tissue.
Conventional DOT and FMT systems include a light source (such as a diode laser and appropriate driver), and an optical switch, which provides light to a group of optical pathways, for example, optical fibers. The optical switch directs the light source to selected ones of the optical fibers, one at a time, in a sequence. The optical fibers are in direct contact with a diffuse medium to be imaged. Using the optical fibers, the single laser source is directed to selected points on the surface of the diffuse medium. Light is collected with the use of fiber bundles, placed at multiple points, also in direct contact with the surface of the diffuse medium, and the light is directed though the fiber bundles from the diffuse medium to appropriate light sensors. A computer performs tomographic data transformations to provide images for display and storage.
Existing DOT and FMT systems employ light sources and light sensors in direct contact with the object to be imaged, providing direct contact systems. For a description of an exemplary optical tomography system, see D. J. Hawrysz and E. M. Sevick-Muraca, “Developments Toward Diagnostic Breast Cancer Imaging Using Near-Infrared Optical Measurements and Fluorescent Contrast Agents,” Neoplasia, vol. 2, pp. 388-417, 2000.
The contact light sensors each receive light essentially from a single respective point on the surface of the object. Direct contact systems tend to reduce system versatility, limiting the shapes, sizes, and geometries of objects that can be tomograhically imaged with any particular DOT or FMT system. Direct contact systems, when used to image body parts of a patient, also tend to limit patient comfort.
Some optical tomography systems can only be used to image objects having a particular shape. For example, Wake et al., U.S. Pat. No. 6,211,512, describes an optical tomography system for use only with objects having a cylindrical geometry.